Use of tantalum powders for the preparation of anodes in electrolytic capacitors is generally well known. Such anodes (electrodes) are produced by compacting the tantalum powder to provide a coherent shape, sintering the compact, and subsequently forming a continuous dielectric oxide film on the sintered product. In such capacitors, it is generally desirable to have as high a specific charge or capacity (CV/g or .mu.fv/g) as possible. U.S. Pat. No. 3,418,106 discloses an agglomerated tantalum powder crushable as tantalum which, when fabricated into an electrode, provides enhanced specific capacity in pressed and sintered anodes. U.S. Pat. No. 3,843,360 discloses an improved tantalum powder in which the relationship between the average particle diameter of the powder and its bulk density has been limited to a specific range. In U.S. Pat. No. 4,347,084, an electrode having a specific capacity in excess of 10,000 .mu.fv/g is prepared by sintering a tantalum powder having a grain size finer than 2.5 .mu.m.
Another approach toward achieving improved specific capacity in tantalum powders for electrolytic capacitors is to modify the particle shape and morphology. In U.S. Pat. No. 3,647,415, a tantalum powder for electrolytic capacitors is disclosed wherein the tantalum particles have an average flakiness of 2 to 60, at least 80 percent by weight of the powder consisting of particles having a shorter breadth of 3 to 250 .mu.m and being free from particles having a shorter breadth of about 500 .mu.m.
Additives (or dopants) to the tantalum powder have been proposed to provide (or help retain) higher specific capacity than the undoped powder. U.S. Pat. No. 4,009,007 discloses a tantalum powder capable of producing anodes of improved electrical capacity which is prepared by the addition of phosphorous-containing materials in amounts from about 5 to about 400 ppm based on elemental phosphorus. Such phosphorous-containing additives can be made to the powder itself, as in the above U.S. Pat. No. 4,009,007; or the dopant can be added, as described in U.S. Pat. No. 4,356,028, during the method of preparing tantalum powder from a tantalum-rich solution in an organic solvent in which a tantalum salt is precipitated from the tantalum rich solvent and the tantalum salt is reduced to metallic tantalum by an alkali metal, wherein a phosphorous-containing material is added in either the precipitation step or the reduction step, or both, and in which the final tantalum powder contains from about 2 to 400 ppm phosphorous on a tantalum metal basis. U.S. Pat. No. 4,379,128 discloses that alkali tantalum fluoride salts, having improved properties as a source for tantalum metal powder, may be produced by the introduction of a phosphorous-doping agent to the salt during its preparation.
German Publication No. DE 3140248A1 describes a process for the manufacture of a porous metal (tantalum and/or niobium) powder doped with boron or boron compounds for electrolytic capacitors having lower relative leakage and higher specific capacity. This publication states that the doping with boron or boron compounds, in quantities up to 0.5 weight percent based on the metal content, can be carried out during the manufacture of the powder, or be added to the green porous metal anodes.
In U.S. Pat. No. 3,825,802, particles of tantalum constituting a porous electrode were doped with a dopant selected from the group consisting of nitrogen, silicon, boron and mixtures thereof. For nitrogen, the dopant ranges from 0.47 to 2.71 atomic percent (or 0.5 to 3.0% TaN added).
U.S. Pat. No. 4,154,609 describes a method of producing a porous sintered metal body which comprises molding under pressure metal powder to form a molded body and then sintering the molded body, with the metal being selected from the group consisting of Groups IVB, VB, and VIB of the Periodic Table. Prior to the molding step, the metal powder is admixed with about 0.01 to about 10% by weight of at least one inorganic lubricant selected from sulfides, selenides and tellurides of a member of sub-groups V and VI of the Periodic Table, whereby the sintered metal body still contains 2% to 70% by weight of the inorganic lubricant. U.S. Pat. No. 4,229,217 discloses a method of producing a porous sintered metal body which comprises the steps of molding metal powder under pressure to form a molded body and then sintering the molded body, with the metal powder being selected from the group consisting of Groups IVB, VB, and VIB of the Periodic Table. Prior to the molding step, the metal powder is mixed with about 0.01 to about 10% by weight of at least one inorganic lubricant selected from nitrides of boron and silicon, whereby the sintered metal bodies still contains 2% to 70% by weight of the inorganic lubricant.
The oxygen concentration of tantalum powder destined to be used in making electrolytic capacitors is important, according to the discussion in U.S. Pat. No. 4,423,004. It is stated that, when the total oxygen content of porous tantalum pellets is above approximately 4000 ppm, capacitors using such pellets as electrodes have unsatisfactory life characteristics. Tantalum powder has a great affinity for oxygen, and the powder particles readily oxidize upon exposure to air, particularly after a thermal treatment step.